Paper base lithographic printing plates and methods for making the same have been well known for a considerable period of time. Lithography depends upon the immiscibility of a greasy lithographic printing ink and an aqueous etch or lithographic solution. In use, a paper lithographic printing plate is first imaged in a known manner with typed, written, or drawn copy material to be reproduced. The image may also be obtained in other ways, for instance, by xerography; e.g. Electrofax (Trademark, Radio Corp. of America) and Xerox (Trademark, Xerox Corp.). The grease-receptive imaging material employed makes the imaged areas ink-receptive and water-repellent (i.e. hydrophobic). The remaining non-imaged surface is water-receptive and ink-repellent (i.e. hydrophilic).
The imaged plate is placed on a plate cylinder of an offset duplicating press. The overall surface of the plate then is treated with an aqueous wet-out liquid which wets all portions of the plate except those areas that have been imaged and are water-repellent. The press inking roll then passes over the surface of the plate and deposits a film of ink only upon the ink-receptive imaged areas. In the printing operation, the ink from the imaged areas is transferred in reverse to a rubber offset blanket which in turn prints directly onto a paper sheet so as to form a copy.
Desirable attributes of a lithographic or planographic printing plate include the producing of clean copy, good toning, good imaging, stop-go properties, and lack of curling or wrinkling of plates while on a press. It is also important that the plate, while accepting ink and etch, be sufficiently water resistant so that it will not "milk" or "pick".
Lithographic printing plates of the type described having a coating of positively charged colloidal silica and insolubilized hydrophilic polymer, conventionally require the application of a substantial barrier cost onto the paper base followed by the application of such silica containing coating, which is commonly called a face coat. Such barrier coat containing a pigment and resin binder provides both surface smoothness and water resistance to the paper base sheet. The barrier coat may be dispensed with for a short run plate, but for medium-to-long run plates (e.g. 1,000-5,000 copies), fairly heavy barrier coats, such as those shown in Ser. No. 493,528, advantageously are employed.